I gave an image link below to use as a demenstration.

Would angle BAC be coungruent to angle DAC since they're on a straight line (180 deg.) and angel DAC has been marked as a right triangle (90 deg.) and since 180-90=90 deg. left for angle BAC? Thus, making them congruent? Is this a valid proving process? What theorems or postulates would I use?

http://img192.imageshack.us/img192/9476/46932074.png